1. Field of the Invention
The present invention relates to an optical fiber coupling system and, more particularly, to an optical fiber coupling system with a compact size capable of minimizing a beam loss and suitable for a mass production, and its manufacturing method.
2. Description of the Background Art
Recently, as demands on a high output laser light are increasing and a new application field is extended, researches on a high output semiconductor laser are actively ongoing.
The application field is extended to a display using laser such as a CD-RW or a DVD-RAM, optical recording medium, or a large-screen projector, a laser wireless communication, a material processing using laser (welding, cutting and fine processing related to a semiconductor, or the like), an optical amplifier and a pumping light source used for a medical use or a military use.
In the case of the pumping light source of a solid state laser, the material processing and the medical laser, a high laser output more than a few Watt level is required. Especially, the pumping of the solid state laser is a critical and big application field for a high output diode laser.
One method for obtaining such a high laser output is that a high output laser diode is formed in an array type for use. In this method, in order to heighten an efficiency of the laser diode array, several laser beams radiated from the laser diode array are coupled to each optical fiber corresponding to each laser diode, and the optical fiber array is bundled to be used as a light source for pumping, whereby a high laser output can be obtained with the beam emitted from the laser diode array.
The high output semiconductor laser is used as an optical fiber coupling system according to applications, and the optical fiber coupling system includes an optical fiber array having a suitable numerical aperture (NA) and a core diameter, and a refractive optical lens serving to collimate the diode array beam.
The existing optical fiber coupling system including the refractive optical lens is a device designed for a beam form of a multi-mode high output diode laser. In order to reduce radiation of the laser optical source and couple the optical fiber, the existing optical fiber coupling system requires a complicate three-dimensional shaped lens structure and an optical system with a high performance.
A spread angle of light radiated from the laser diode is determined depending on a structure of the laser diode. In case of the high output laser diode, generally, an light-spread angle is in the range of about 32˜44 in a vertical direction, and about 8˜12 in the horizontal direction. This is because the size of a transversal section of a semiconductor laser oscillation region is a few micrometer (μm), that is, so small that the output light is radiated at a wide angle owing to a diffraction effect.
At this time, having different horizontal and vertical angles, an overall distribution of the horizontal mode of the output light is oval.
In order to reduce the loss of the laser beam made incident on the optical fiber of the output terminal due to such unbalance of the light spread angle, preferably, a laser beam spread angle is to be reduced. In this case, by coupling the optical system utilizing predetermined lenses providing a focusing performance corresponding to each spread direction, the light spread angle can be reduced to obtain a high efficiency for the output terminal optical fiber.
As to the optical connection between the laser diode and the optical fiber of the output terminal, the light spread angles are different at the horizontal and vertical directions, so two lens surfaces are required.
FIGS. 1 and 2 show beam paths from a lens to an optical fiber of the output terminal according to a difference in the light spread angle in the horizontal and vertical directions of the general laser diode in a coupling optical system using a three-dimensional shaped lens in accordance with a conventional art.
With reference to FIG. 1, a lens R1 serves to collimate the laser beam with a wide light-spread angle radiated from the laser diode in a horizontal direction, and with reference to FIG. 2, a lens R2 focuses laser beam radiated from the laser diode 10 in a side direction and makes it incident on a core, or a waveguide, of the optical fiber 20 of the output terminal.
By using the three-dimensional lens, the optical fiber coupling system converts the form of beam radiated from the laser diode. Consequently, the lens R1 having a high numerical aperture collimates from an axis with a wide light-spread angle while the second-dimensional lens R2 projects an expanded light source to an axis with a smaller light-spread angle of the laser diode.
A beam forming function of the optical fiber forms an almost circular spot at an inlet part of the optical fiber of the output terminal and can be used to pump laser mode. An efficiency of the structure is dependent upon the diode laser and the optical fiber as well as the optical system.
In order for the optical fiber coupling system to have a maximum performance, the laser spread angle needs to be small as possible while the aperture of the optical fiber needs to be large, and the lenses of the optical system should be accurately aligned according to size. Light coming into the optical fiber with a small angle is totally, internally reflected from a core-cladding interface, and at this time, a portion of the totally-reflected light is absorbed, causing an output reduction.
Thus, the optical fiber coupling optical system needs to be designed to accomplish a maximum coupling efficiency, and a light source including the optical fiber needs to be optimized. Lenses serving for collimation reduce a spread angle of laser beam radiated from the laser diode, and the smaller the diameter of the collimated beam is, the easier a laser pumping is at a solid state laser medium, which makes the number of the optical devices reduce and the energy density for pumping increase.
In case of the optical fiber used by being optically connected to the high output laser diode array, its numerical aperture is 0.16˜0.39, and its core size is 100˜600 um. Especially, the laser diode emitting light with the optical fiber having a small numerical aperture is significantly used for a medical, solid state laser pumping. A coupling optics used for such usage is fabricated as a product by molding a lens made of glass and replicating it.
In these methods, a high-priced lens is aligned with and attached to the laser diode, the collimated beam coming out through the lens is focused to the optical fiber after passing through several lenses or respectively focused to the optical fiber array positioned at regular intervals, to obtain a high output beam.
However, the two methods are to use a high-priced lens having a complicate three-dimensional shape or a series of lens array, and in this respect, the three-dimensional lens are can be favored for a mass-production because it is fabricated in an injection form using a precisely-processed high-priced metal mold. But fabrication of the metal mold is difficult and costly, and in addition, since the minimum size of the lens is limited to the minimum size of a tool for processing a curved surface of the lens of the metal mold, it is very difficult to make the coupling optical system very small (slim).